Steel Welded Tube Mill – Precision Welding and Rigorous Quality Control

Shijiazhuang Faith Machinery Co.,Ltd’s steel welded tube mill offerings cater to applications where welded tubing must meet exacting dimensional and mechanical specifications. From furniture and construction supports to hydraulic cylinders and automotive components, welded steel tubes produced on these mills deliver consistent performance in demanding environments.

1. Steel Welded Tube Mill Configuration

A typical steel welded tube mill designed by Shijiazhuang Faith Machinery Co.,Ltd incorporates the following modules, optimized for handling carbon or alloy steel strip:

1. Decoiler and Strip Conditioning

• A hydraulic mandrel decoiler supports coils up to 20 tons, with power‐assisted expansion to maintain constant strip tension.

• The strip enters a tension leveling unit with multi‐bar work rolls that eliminate residual stresses and camber, providing a flat, stable entry into the forming section.

2. Forming Stands

• Progressive roll stands, typically 8–12 stands, bend the strip from flat to open U‐shape and then to a closed cylindrical profile.

• Forming rolls are furnished in matched sets, machined for minimal runout (<0.01 mm) to reduce ovality. Roll surfaces are heat treated and ground to enhance wear resistance when processing abrasive high‐carbon or HSLA steels.

3. Welding Section

• ERW (Electrical Resistance Welding): Most steel welded tube mills use ERW technology, applying high‐frequency current (300 kHz to 1 MHz) to heat strip edges. Induction power supplies, featuring insulated gate bipolar transistor (IGBT) converters, deliver stable, programmable current profiles tailored to material thickness and grade.

• Weld rolls press edges together immediately after induction heating, forging a solid‐state bond with controlled flash. Rolling pressure is regulated via servo or hydraulic actuators to minimize edge thinning and ensure proper fusion.

4. Weld Bead Treatment

• Optional inner and outer scarfing units remove residual flash from the weld seam, critical for hydraulic or instrument tubing where internal smoothness is required.

• Automated bead removal heads, equipped with rotary cutters and vacuum extraction, minimize manual trimming and maintain a clean production environment.

5. Sizing and Calibration

• Sizing stands refine tube outer diameter and roundness to meet tolerances (commonly ±0.5%). Laser micrometers positioned before and after the sizing section measure tube geometry at multiple points around the circumference, feeding back to the PLC for dynamic roll adjustment.

• For tubes requiring higher precision (e.g., mechanical tubing for automotive applications), an additional stretch‐forming pull‐off module applies axial tension to correct ovality and improve straightness (targeting <1 mm deviation over 3 m).

6. Cutoff and Finishing

• Flying cold saws, driven by servo motors, produce tube lengths up to 12 m with ±5 mm accuracy at line speeds up to 80 m/min.

• Hydraulic or pneumatic cutoff shears provide a cost‐effective option for lower‐speed, heavier‐gauge tubes, delivering burr‐free ends suitable for secondary machining.

• Downstream end‐forming units (e.g., chamfering, cold expansion) complete tube preparation for downstream joining or assembly.

2. Material Capabilities and Tube Specifications

Shijiazhuang Faith’s steel welded tube mills support a broad spectrum of steel grades and tube dimensions:

• Carbon Steel Tubes:

• Diameters from 15 mm to 219 mm; wall thicknesses from 0.8 mm to 6 mm.

• Common grades: ASTM A500, EN 10219, and structural grades such as S235JR and S355JR. Applications include furniture frames, handrails, and general structural supports.

• High‐Strength Steel Tubing:

• HSLA (High Strength Low Alloy) grades and microalloyed steels (e.g., ASTM A572, A606) require precise weld parameter control to achieve targeted mechanical properties (yield strength 345 MPa to 485 MPa).

• Additional cooling modules (water quench or forced‐air) may be integrated to refine the weld zone microstructure, ensuring toughness and preventing hardness spikes.

• Alloy Steel Tubing:

• Medium‐alloy steels (e.g., 4140, 4340) used in mechanical and automotive applications, demanding narrow weld heat‐affected zones (HAZ) and tight hardness control.

• Induction or pendant post‐weld cooling units help tailor weld zone hardness to match the parent material, often followed by tempering ovens to relieve residual stresses.

• Stainless Steel Tubes (Optional):

• Though primarily focused on carbon and alloy steels, certain welded tube mill lines can process stainless steels (304, 316, 321) up to 3 mm wall thickness. Special passivation and cleaning stations ensure corrosion resistance post‐welding.

3. Automation, Control, and Quality Assurance

To meet the rigorous requirements of steel welded tube production, comprehensive automation and control systems are integrated into every mill:

• PLC and HMI Integration:

• A distributed PLC network (e.g., Siemens S7 or Rockwell ControlLogix) coordinates all modules—decoiler, leveler, roll stands, welding power supply, and cutoff.

• The HMI displays real‐time process metrics, including coil ID, strip thickness, weld current/voltage, tube diameter, wall thickness, and machine speeds. Operators select recipes for different tube sizes and steel grades, with each recipe storing over 50 parameters including forming roll positions, welding power profiles, and tension setpoints.

• Weld Seam Inspection:

• Surface Inspection: High-resolution cameras capture the weld bead appearance; image‐processing algorithms detect bead irregularities, undercut, or excessive flash.

• Inline Ultrasonic Testing (UT): A phased-array UT system scans the entire weld seam at production speed, detecting internal defects such as lack of fusion, porosity, or slag inclusion. The system automatically tags and diverts defective tube sections without halting the line.

• Eddy-Current Testing (ECT) (for alloy grades): An ECT probe inspects surface and near‐surface anomalies in alloy or coated steels, ensuring compliance with tight quality standards.

• Dimensional Control:

• Laser micrometers measure tube outer diameter at multiple points; deviations beyond ±0.5% prompt automatic roll gap adjustment.

• Ultrasonic wall-thickness gauges (optional) verify tube wall thickness immediately after welding, especially important for thicker or high-strength materials requiring consistent wall uniformity.

• Tension and Speed Synchronization:

• VFD-driven motors on the decoiler, leveler, and pull-through modules maintain constant strip tension to avoid seam misalignment.

• Encoders on pull rolls and forming stands ensure roll speeds match strip feed, preventing strip buckling or seam buckling.

• Data Logging and Traceability:

• All process data—including coil heat numbers, weld parameters, inspection results, and dimensional readings—are archived in a centralized database.

• Production managers generate batch traceability reports, enabling root cause analysis when quality issues arise and supporting customer audits.

4. Customization and Layout Options

Shijiazhuang Faith Machinery Co.,Ltd recognizes that each steel welded tube mill installation has unique production goals and plant constraints. Customization options include:

• Forming Roll Configurations:

• Fixed-gap stands for simple diameter ranges (e.g., 33.4 × 2 mm to 60.3 × 3 mm), with manual shim adjustments.

• Servo-driven stands for rapid, motorized roll gap setups, reducing changeover times for frequent size variations.

• Weld Head Selection:

• Single-head ERW for moderate speeds (up to 60 m/min) and common diameters.

• Dual-head ERW (tandem welding) for higher throughput on standard sizes, achieving weld speeds up to 120 m/min.

• SAW heads for larger diameter tubes (above 114.3 mm) or thicker wall gauges (above 6 mm), providing greater weld penetration and stable bead geometry.

• Cooling and Heat Treatment Modules:

• Water quench boxes immediately downstream of the weld seam help refine grain structure and control HAZ hardness, particularly for HSLA and alloy grades.

• Temperature-controlled cooling rolls prevent thermal distortion in sensitive materials.

• Cutoff Options:

• Flying cutoff saws (for continuous high-speed operation) up to 80 m/min produce lengths up to 12 m with ±5 mm accuracy.

• Hydraulic cutoff shears (for heavy gauge) produce burr‐free ends suitable for bending and threading.

• Downstream Finishing:

• Automated end chamfering units prepare tube ends for welding or connection.

• Cold expansion units locally strengthen the weld zone for high‐pressure applications.

• Inline hydraulic or pneumatic straighteners further improve tube straightness, achieving <2 mm deviation over 6 m.

• Layout Configurations:

• Straight-line layout for facilities with sufficient length, minimizing transfer distances between modules.

• L-shaped or U-shaped layouts for plants with limited linear space; modules are positioned to optimize floor usage while maintaining process sequence.

5. After-Sales Support and Maintenance

Ensuring sustained performance and minimal downtime, Shijiazhuang Faith Machinery Co.,Ltd offers comprehensive support services:

• Turnkey Installation and Commissioning:

• Site preparation guidance, including foundation design, civil works specifications, and utility requirements (power, compressed air, cooling water).

• Mechanical alignment, PLC wiring, and system calibration are conducted by Faith Machinery’s engineers, followed by trial production runs to validate performance.

• Operator and Maintenance Training:

• Hands-on training covers safe operation, changeover procedures, and preventive maintenance tasks such as roll inspection, bearing lubrication, and welding power supply checks.

• Detailed operation manuals and maintenance schedules are provided, along with suggested spare parts lists.

• Spare Parts and Consumables:

• Recommended spare parts kits include form roll bearings, welding power capacitors, hydraulic seals, blade sets for scarfing and cutoff, and sensor modules.

• Customers can choose from standard parts kits or tailor kits based on their production volume and critical wear points.

• Remote Diagnostics and Software Updates:

• Secure remote access enables Faith Machinery’s technical team to review PLC logs, adjust weld parameters, and troubleshoot alarms without on-site visits.

• Periodic software updates ensure compatibility with the latest HMI features, data-logging standards, and security protocols.

• Field Service and Retrofits:

• Scheduled field service visits include alignment checks, weld head tuning, and inspection of NDT equipment.

• Retrofit options—added forming stands, higher-capacity weld power supplies, advanced NDT modules—allow existing tube mill lines to accommodate new materials or increase throughput.

By combining durable mechanical design, precise welding control, and comprehensive quality assurance, Shijiazhuang Faith Machinery Co.,Ltd’s steel welded tube mills enable producers to manufacture welded tubes that meet stringent industry standards for strength, dimensional accuracy, and surface finish.